Nitrogen Dioxide Removed Annually by Tree Cover This EnviroAtlas community map estimates the total kilograms of nitrogen dioxide removed annually by trees in each census block group. Why is nitrogen dioxide removal important? Nitrogen dioxide (NO2) is the indicator component for the larger group of gases known as nitrogen oxides (NOx). NO2 forms very quickly in the atmosphere, largely from mobile 1 sources, power plants, and off-road equipment. NO2 is a common air pollutant, and it is one of the six criteria pollutants regulated by EPA under the National Ambient Air Quality Standards (NAAQS). NO2 affects air and water quality, contributes to the formation of important greenhouse gases, and negatively affects respiratory health. Trees are capable of removing NO2 from the atmosphere, thus contributing to air and water quality, climate stabilization, and public health. associated environmental and climatic effects.1 Ozone and methane (CH4) are two potent greenhouse gases (GHGs) that Nitrogen dioxide, along with other air pollutants, can have warm the atmosphere. Thus, the removal of NO2 may significant effects on human health, including aggravation of contribute to a more stable climate by helping to reduce the respiratory and cardiovascular diseases, decreased lung amount of GHGs in the atmosphere. function, increased frequency and severity of respiratory symptoms (e.g., difficulty breathing and coughing), and Trees help reduce the potential adverse health and 1 increased susceptibility to respiratory infections. People with environmental effects of NO2 by removing it from the air. pre-existing conditions such as heart disease, asthma and Gaseous air pollutants are taken in primarily through the leaf emphysema, as well as older adults and children, are at greater stomata (pores), though some gases are removed by the plant risk for air pollution-related health effects. Despite surface. Once inside the leaf, gases diffuse into intercellular improvements in air quality, approximately 127 million spaces and may be absorbed by water films to form acids or people live in areas that exceeded the NAAQS in 2008.1 react with inner-leaf surfaces.3 The removal of gaseous Additionally, the number of people with asthma is growing, pollutants is more permanent than the removal of particulates with about 1 in 12 people (25 million, 8% of U.S. population) because the gases are often absorbed and converted within the having asthma in 2009, compared with 1 in 14 (about 20 leaf interior. Healthy trees can remove significant amounts of million, or 7%) in 2001.2 air pollution in cities, where it is often concentrated.

In addition to its potential health effects, NO2 also has How can I use this information? environmental impacts. Nitrogen oxides contribute to the 2 The map, Nitrogen Dioxide Removed Annually by Tree acidification and nutrient enrichment of surface water. Cover, estimates and illustrates the variation in the amount of Deposition of nitrogen can lead to fertilization, air pollution removed by trees. These data could be used to eutrophication, or acidification of terrestrial, wetland, and explore the patterns of NO2 removal by trees in communities aquatic (e.g., fresh water bodies, estuaries, and coastal water) 1 that do not meet the standards set by the EPA’s NAAQS. For systems, potentially affecting water quality. Thus, cleaner air compliant areas, the map can identify neighborhoods that means fewer pollutants that can redeposit onto land and water potentially have higher NO2 concentrations compared to other bodies, degrading water quality. neighborhoods. When used with EnviroAtlas data and maps Air pollution affects the Earth’s climate by either absorbing that look at near-road environments, users can explore areas or reflecting energy that can lead to climate warming or where high percentages of the block group population are in close proximity to roadways that have notably low volumes cooling, respectively.NO2 contributes to the formation of ground level ozone, methane, and particles, which have of NO2 removed by trees. Additionally, communities and

CONTINUED ON BACK 1 researchers that have access to health data may be able to use small impact on the estimate. Additionally, this map uses this map and its underlying data to continue to research the weather and pollutant monitoring data to represent local relationships among trees, nitrogen dioxide, and human conditions, though a city’s average weather and pollutant health. conditions do not depict potential variability of conditions within the community. How were the data for this map created? The data for this map are based on the land cover derived for How can I access these data? each EnviroAtlas community and the pollution removal EnviroAtlas data can be viewed in the interactive map, models in i-Tree, a toolkit developed by the USDA Forest accessed through web services, or downloaded. To find the Service. The land cover data were created from aerial EnviroAtlas 1-meter land cover grids created for each photography through remote sensing methods by which tree community, enter land cover community in the interactive cover was summarized as the percentage of each census block map search box. group. The i-Tree pollution removal module uses the tree cover data by block group, the closest hourly meteorological Where can I get more information? monitoring data for the community, and the closest pollution There are numerous resources where additional information monitoring data. Additionally, the 2001 National Land Cover on nitrogen dioxide as an air pollutant can be found; a Dataset (NLCD) was used to determine the percent of trees selection of these resources is listed below. For information that were deciduous or evergreen. Local leaf-on and leaf-off on EPA air pollution rules, regulations, and monitoring dates were used to vary canopy cover daily based on the programs, please visit the Agency’s website. To learn more amount of tree cover classified as deciduous. Assuming a leaf about i-Tree tools and how they can be used to support area index value of 4.9, hourly estimates of pollution removal research, planning, and policy efforts, visit the i-Tree website. by trees were combined with atmospheric data to estimate For more information on how air pollution and its removal hourly percent air quality improvement due to pollution may affect human health, visit the Clean Air section of the removal for each pollutant.4 Eco-Health Relationship Browser. For additional information on the data creation process, access the metadata found in the What are the limitations of these data? layer list drop-down menu for map layers in the EnviroAtlas All of the EnviroAtlas community maps that are based on land interactive map. To ask specific questions about these data, cover use remotely-sensed data. Remotely-sensed data in please contact the EnviroAtlas Team. EnviroAtlas have been derived from imagery and have not been verified. These data are estimates and are inherently Acknowledgments imperfect. This map also uses estimation methods for EnviroAtlas is a collaborative effort by EPA, its contractors, pollution removal. To accomplish this, average leaf area index and project partners. The data for this metric were generated values from urban areas were used. These averages may not by Satoshi Hirabayashi and Allison Bodine, Davey Tree accurately reflect local conditions, but since local values are Expert Co. and David J. Nowak, USDA Forest Service. The not available, these are the best usable estimates. This fact sheet was created by David J. Nowak, USDA Forest limitation is not particularly significant because leaf area Service, Jessica Daniel, EPA Student Services Contractor, index values do not vary substantially and have a relatively and Laura Jackson, EPA.

Selected Publications 1. United States Environmental Protection Agency (EPA). 2017. Our nation’s air: Status and trends through 2016. Accessed October 2020.

2. Centers for Disease Control and Prevention (CDC). 2012. Asthma in the U.S: Growing every year. Accessed October 2020.

3. Smith, W. H. 1990. Air pollution and forests. Springer-Verlag, New York, 618 p.

4. Nowak, D.J., D.E. Crane, and J.C. Stevens. 2006. Air pollution removal by urban trees and shrubs in the United States. Urban Forestry and Urban Greening 4:115–123.

EnviroAtlas: Led by the U.S. Environmental Protection Agency July 2020